1. Field of the Invention
The present invention relates generally to miniaturized turbomachines, and, more particularly, to a miniature gas turbine engine for power generation with a unitary rotor shaft where turbine, compressor, rotor shaft, and power generator are integrated in one piece.
2. Description of the Related Art
The fundamental components of a gas turbine engine for power generation include a combustor, a turbine, a compressor, a rotor shaft, and an alternator, which is on the extension of the rotor shaft and which converts mechanical energy into usable electric power. The turbine and compressor reside in a high temperature (hot) zone of the engine while the rotor shaft, carrying the turbine and compressor, extends from the hot zone to a low temperature (cold) zone. In known gas turbine engines, the turbine, compressor, and rotor shaft are usually joined or assembled and not fabricated as one piece. Furthermore, the rotor shaft may be made of two different materials such as steel and ceramic.
In U.S. Pat. No. 4,063,850, titled “GAS TURBINE ENGINE HAVING A CERAMIC TURBINE WHEEL,” issued to Hueber et al., and assigned to Motoren- und Turbinen-Union Munchen GmbH, Munich, Germany, hereinafter referred to as Hueber, a rotor shaft comprising a metal portion and a ceramic portion is disclosed. In Hueber, a turbine wheel is made from a ceramic material and is formed integrally as one piece with the ceramic portion of the rotor shaft. The place of connection, which may be made by brazing, interlocking, or fusing, of the metal portion to the ceramic portion of the shaft is located in a cooler zone of the engine. Precision shaft assembly is required to manufacture the gas turbine engine disclosed by Hueber.
In U.S. Pat. No. 5,102,305, titled “TURBOMACHINE HAVING A UNITARY CERAMIC ROTATING ASSEMBLY,” issued to Bescoby et al., and assigned to Allied-Signal Inc., N.J., U.S.A., hereinafter referred to as Bescoby, a unitary ceramic rotating assembly mounted within a split housing is disclosed. In Bescoby, the unitary rotating assembly includes a ceramic shaft, fins, a bearing runner, as well as a turbine and a compressor symmetrically attached to opposite ends of the ceramic shaft, all of which are mounted within the split housing. The unitary rotating assembly of Bescoby is a complex design where the rotor shaft must be mounted within the split housing and supported by a hydrodynamic film of fluid. Furthermore, Bescoby does not address whether the unitary rotating assembly is suitable for generating power in miniature applications.
As illustrated in Hueber and Bescoby, conventional compressor and turbine for a gas turbine engine have three-dimensionally complicated geometry based on turbo machinery design. Due to design and shape complexities, individual components are manufactured separately and assembled by using a various kind of joining techniques. For miniature applications, such conventional assembly process becomes more challenging as the size of the components is scaled down.
The need for a portable miniature power source is increasing with the development of unmanned aerial vehicles, autonomous robots and other scaled-down mobile, sensory, surveillance devices. As one skilled in the art would appreciate, since the physics and mechanics influencing the design of the components of a miniature device do change with scale, some scaling effects in designing miniature devices can be extremely difficult to overcome, for example, the viscous forces in the fluid, surface area to volume ratios, chemical reaction times, the electric field strength, and manufacturing constraints, which are limited mainly to two-dimensional planar geometries. Other design and aerodynamic issues related to the compressor and turbine of miniature turbomachines include low Reynolds number, surface roughness, tip clearances, and minimum feature size, all of which can lead to reduced levels of performance.